I forget where the link is (it's somewhere on this forum) but apparently the version of Amavisd-new in Zimbra has a minor bug. It WILL CONTINUE to place copies of the emails in the quarantined location, although the final bad header destiny is configured to allow the email to pass.

All I did was left things as they were and for safety's sake increase the time for which quarantined emails are left in the cron job: